1. Field of the Invention
The present invention relates to a vessel for a high pressure discharge lamp and a method of manufacturing the same. The present invention also relates to a high pressure discharge lamp having such a vessel and a method of manufacturing the same.
2. Description of the Related Art
Such a vessel is generally classified into two types. The vessel according to a first type is called as “integrated type vessel” and has a main portion forming a discharge space and end portions integrated into the main portion. The vessel according to a second type is called as “assembled type vessel” and has a main portion and separate end portions which are inserted into the respective openings of the main portion and thereby assembled with the main portion. However, the assembled type vessel cannot be used for a low watt type of high pressure discharge lamp because of a low lamp efficiency due to the heat loss at junctions of the main portion and the respective end portions. The assembled type vessel cannot be used for a high pressure discharge lamp either, because lamp efficiency is an important factor even for a middle-high watt type of high pressure discharge lamp. Therefore, when such lamps are to be manufactured, it has been considered necessary to use the integrated type lamp which does not suffer from the above-mentioned disadvantage of the assembled type vessels.
It is desirable that the transmittance of the lamp is as high as possible, so that at least a central area of the main portion of the vessel should be as thin as possible. On the other hand, it is desirable that the mechanical strength of the end portions to be inserted by the respective electrode members is as high as possible, so that the thickness of the end portions should be as large as possible. Also, as a light-emitting material tends to be collected and the proceeding of corrosion is fast in the neighborhood of boundary areas between the respective end portions and the main portion, it is preferable that the thickness of the neighborhood is as large as possible to mitigate adverse influence of corrosion and achieve prolonged lifetime. Therefore, by using a vessel having an entirely uneven thickness wherein main portion has a thickness at the central area which is smaller than at the respective end portions and at the boundary areas between the respective end portions and the main portion, it is possible to manufacture the lamp having a prolonged lifetime as compared to the lamp with a vessel having an entirely uniform thickness.
Conventionally, when the integrated type vessel is formed with a blow molding of the vessel as disclosed in JP-A-10-81183, for example, as shown in FIGS. 1A and 1B, a tubular shaped body 1 (FIG. 1A) made of a transparent or translucent ceramic material such as alumina is arranged between an upper half 2 and a lower half 3 of the mold, these mold halves 2, 3 are moved toward each other as shown by arrows a and b, respectively, to set the shaped body 1, and a pressure atmosphere such as air is introduced into an opening 4 of the shaped body 1 so as to obtain a blow-molded body 5 (FIG. 1B) of the vessel.
In the case of the blow molding process, it is possible to manufacture a vessel in which at least the central area of the main portion has a thickness smaller than at the respective end portions and at the boundary areas between the respective end portions and the main portion. However, it is necessary for the opening 4 to have a diameter enough to admit air into the opening 4. As a result, it is difficult for the inner diameter of the respective end portions to have a diameter smaller not more than a designated value of 2 mm, for example. Even if it is possible, it is still difficult for the main body of the vessel to keep a necessary inner diameter of 1–15 mm, for example.
In the case of the casting process disclosed in JP-A-7-107333, for example, as shown in FIGS. 2A to 2C, after a slurry 9 has been introduced into the mold 6 (FIG. 2A) from an opening 7 and coated over the inner surface 8 of the mold 6 (FIG. 2B), the excess slurry 9 is removed so as to obtain a molded body 10 (FIG. 2C).
In this case, it is possible to preserve the opening 7 with a diameter not more than 2 mm (but not less than 0.8 mm), since it is only necessary for the opening 7 to secure a diameter enough to remove the excessive the slurry 9. However, because of the nature of the casting process, it is impossible to form a vessel in which at least the central area of the main portion has a thickness smaller than at the respective end portions and at the boundary areas between the respective end portions and the main portion.